1. Field
This disclosure relates generally to data prefetching and, more specifically to techniques for indirect data prefetching.
2. Related Art
Data prefetch (cache management) instructions, which have used direct addressing, have been employed to move data into a cache before the data is accessed in an attempt to minimize cache-miss latency. Data prefetch instructions (which may be added to software by a compiler or programmer) are generally treated as hints, which affect software performance but not software functionality. Hints can be used to move data into a given cache level in an attempt to provide a desired performance. Prefetched data in a same cache line as desired data has spatial locality and has a high probability of being accessed in the near future. Data with temporal locality, or persistence, is expected to be accessed multiple times and should generally be maintained in a cache for ready accessibility. Accesses to data with no temporal locality are transient and, as such, data with no temporal locality should generally be removed from a cache after use to provide room in the cache for other data.
Some data prefetch instructions allow for specifying a cache level in which data should be stored. Various data prefetch instructions make a distinction between memory which is expected to be read and memory which is expected to be written. When data is to be written, a data prefetch instruction usually moves a data block into a cache so that an expected store is to the cache. A prefetch for a write generally brings data into a cache in an exclusive or modified state. A prefetch for data to be written can usually be replaced with a prefetch for data to be read (this is usually how implementations that define both kinds of instructions, but do not support prefetch for writes, operate). A data block accessed by a data prefetch instruction usually corresponds to a cache line, whose size is implementation specific. Various architectures implement data prefetch instructions with a base update form, which modifies a prefetch address following a prefetch. Base update (pre/post increment/decrement) is also supported on load and store instructions for some architectures and can be taken into consideration in code that uses data prefetch instructions.
Some architectures implement data prefetch instructions that cause faults when the address to prefetch is invalid or not cacheable. In general, data should be in a cache prior to being accessed. However, using prefetches that are too speculative can have negative affects as there are costs associated with data prefetch instructions. Similarly, prefetching data that is already in the cache increases overhead without providing any benefit. For example, data may already be in a cache when the data is in the same cache line as data already prefetched (spatial locality) or if the data has been used recently (temporal locality). Some architectures recognize data prefetch instructions but treat them as no operation (nop) instructions. Usually, data prefetch instructions are ignored for memory pages that are not cacheable. However, a base address of a data prefetch instruction that has a base update form may be updated even if addressed memory cannot be prefetched.